The present invention relates to radial-ply pneumatic tires. Specifically this invention relates to passenger or light truck tires utilizing an annular tread-reinforcing structure having one belt ply of steel cords and a second one of aromatic polyamide fiber cords.
In general, conventional pneumatic radial tires have an annular tread-reinforcing layer or belt package having two belt plies both of which are usually comprised of cords made from the same material, such as steel, fiberglass, etc. However, there has been an attempt lately to produce a tire having a belt package of two different types of belts. For example, U.S. Pat. No. 4,602,666 to Kabe et al (1986) teaches the use of one belt having steel cords and another belt of aromatic polyamide fiber cords having its side ends bent or folded over toward each other. Another attempt has been to place the aromatic polyamide fiber cords radially inwardly from the steel cords and then to provide for strips to overlap the ends of the steel belt as shown in U.S. Pat. No. 4,184,530 to Mirtain (1980). Neither of these patents produces a tire having only a single belt ply of steel and a single belt ply of aromatic polyamide. Furthermore, using two different types of belt plies can produce various problems.
It is believed that the difference in the bending stiffness between the steel cord and the aromatic polyamide cord causes extra stress within the tire. As used herein bending stiffness is the resistance of the belt material to forces which cause both in plane and out of plane bending and is expressed in terms of force per unit area, e.g. lbs/in.sup.2. This stress may manifest itself in premature or uneven tread wear and/or a reduction in durability performance, such as premature tire failure due to belt separations. It is believed that these problems can be reduced by equalizing the stiffness of the different types of cords.
The cords of each belt make an angle with respect to the mid-circumferential centerplane of the tire. It is believed that the stiffness of the aromatic polyamide cords can be increased to more approximate that of the steel by reducing the angle that they make with respect to the mid-circumferential centerplane. In other words, the angle which the aromatic polyamide cords make should be less than the angle which the steel cords make with respect to the mid-circumferential centerplane. Therefore, the stiffness of both belts can be adjusted to produce a more equal result. The mathematics of the above, i.e., whereby the stiffness of the aromatic polyamide cord increases with a reduction in cord angle, is outlined by the equations for Youngs Modulus as a function of cord angle in Mechanics of Pneumatic Tires by J D Walter and incorporated as a reference herein.
Furthermore, as used herein, including the claims, the following terms are defined as follows:
"Axial" and "axially" refer to directions and/or displacements that are parallel to the axis of rotation of a tire.
"Bead" means that part of the tire comprising an annular tensile member wrapped by ply cords and shaped to fit the design rim.
"Carcass" means the tire structure apart from the belt structure, tread, undertread, and sidewall rubber over the plies, but including the beads.
The "mid-circumferential centerplane" of a tire is a plane that is perpendicular to an axis of rotation of a tire and which is located midway between the external surfaces of the sidewalls of a tire at its maximum axial width exclusive of ornamentation or indicia.
"Radial" and "radially" refer to directions and/or displacements from the axis of rotation of the tire.
"Radial-ply tire" means a pneumatic tire in which the ply cords which extend from bead to bead are laid at cord angles between 75 degrees and 90 degrees with respect to the mid-circumferential centerplane of the tire.
"Sidewall" means that portion of a tire between the tread and the bead.
"Tread" means that portion of the tire that comes into contact with the rod when the tire is normally inflated and under normal load.
"Tread width" means the arc length of the tread surface in the axial direction.